Oxygen/carbon dioxide sensor and controller for a refrigerated controlled atmosphere shipping container

ABSTRACT

A controller for use in a system employing selective permeable membrane technology to maintain a controlled atmosphere within a refrigerated container. The controller is electrically interfaced to an oxygen and carbon dioxide sensing device which is disposed within the container and capable of withstanding severe environmental conditions to measure the levels of oxygen and carbon dioxide within the container. The controller maintains preset levels of oxygen and carbon dioxide within the container and is adapted to calibrate the carbon dioxide and oxygen sensing devices as well as check for proper operation of the sensing devices and default to safety conditions when a failure is detected. The controller is further adapted to control the atmosphere maintenance system in a manner adapted not to increase the peak power requirements of the system.

RELATED APPLICATIONS

The present application is a continuation application of Ser. No.08/603,863, filed Feb. 22, 1996, now abandoned, which is a continuationapplication of Ser. No. 08/374,876, filed Jan. 19, 1995, now U.S. Pat.No. 5,623,105, which is a continuation application of Ser. No.08/113,428, filed Aug. 26, 1993, now abandoned, which is a divisionalapplication of Ser. No. 07/964,937, filed Oct. 21, 1992, now abandoned.

FIELD OF THE INVENTION

The present invention relates generally to the production of controlledatmosphere environments, and more particularly to a device used for thecontrol of oxygen and carbon dioxide levels within refrigerated shippingcontainers used for the transportation of perishable foods such asfruits, produce, meats or grain.

BACKGROUND OF THE INVENTION

The broad concept of utilizing controlled environments to promotestorage life of perishable commodities such as fruits, meats, produceand grains is well known. Though the use of refrigeration units has beena common technique of preserving perishable goods, certain types ofcontrolled atmosphere systems have recently been introduced whichoperate through the controlled manipulation of carbon dioxide and oxygenlevels within transport/shipping containers in addition to the use ofrefrigeration. In this respect, hollow fiber permeable membranes areused for air separation in maintaining controlled carbon dioxide andoxygen levels within grain elevators and controlled atmospherewarehouses. As with most controlled atmosphere systems, those utilizingpermeable membranes require a reliable source of compressed air.Typically, a compressor is used to provide a source of compressed airfor the controlled atmosphere system. Additionally, since it isdesirable to monitor the carbon dioxide and oxygen levels of thecontrolled atmosphere system, such systems typically include a samplingpump which is used to draw conditioned air from the transport/shippingcontainer into carbon dioxide and oxygen sensing devices.

Recent advances in membrane technology have increased the efficiency anddecreased the size of gas-permeable membrane systems thereby making theapplication of membrane technology more feasible for controlledatmosphere transport applications. However, though the technologyassociated with permeable membranes has advanced, transportrefrigeration units typically do not include controlled atmospheredevices because of the reduced cargo space, increased weight, power andcost. Additionally, the corrosive marine environments and extremetemperature parameters typically encountered by mobile transportcontainers makes controlled atmosphere applications in conjunction withsuch containers very difficult.

One example of a prior art apparatus for producing a controlledatmosphere utilizing permeable membranes is disclosed in U.S. Pat. No.4,187,391 issued to ROE. As disclosed therein, the ROE apparatusrequires the use of a controller of high reliability. Existing prior artcontrollers have proven to be inadequate for such purposes because ofthe instability and reliability of the gas analysis components. Further,sampling pumps typically utilized to produce the necessary gas flow forthese analyzers have also proven inadequate and expensive. Further, aprocessor employing complex logic to assign priorities to satisfycontrol parameters detect failure of the sensing components and defaultto a safe best mode operating condition under varying circumstances isrequired to insure produce life within the container. For mobiletransport applications an additional problem encountered in shipping isthat only limited electrical power is typically available for operatingthe system. The present invention specifically overcomes these and otherdeficiencies associated with prior art controlled atmospheric systems.

SUMMARY OF THE INVENTION

In accordance with the preferred embodiment of the present invention,there is provided a controller for a system for maintaining a controlledatmosphere within a sealed container. The system generally comprises acompressor to produce compressed gas and a membrane having different gasdiffusion rates for separating the gas into its components. Thecontroller comprises an analyzer for carbon dioxide (CO2) analysis, ananalyzer for oxygen (O2) analysis, a means to calibrate these analyzers,and a means to circulate sample gases and calibrant gases through theseanalyzers. In the preferred embodiment the controller also has multipleinputs for sensing the temperature of the gas entering the membrane, fordetermining when the refrigeration system of the shipping container isoperating at reduced power and for determining when the refrigerationsystem of the container is in a defrost cycle. The controller employs amicroprocessor to control the temperature of the membrane input gas, abypass valve for the membrane to control the amount of oxygen and carbondioxide gas entering into the container, a valve to introduce carbondioxide to the refrigerated chamber of the container and also controlthe CO2 level therein and a solenoid valve to periodically introducecalibrant gases to the analyzers.

ANALYZER PORTION OF THE CONTROLLER

The 170 degree Fahrenheit temperatures typically produced by thecombined sun load, compressors, and other machinery in the spaceavailable outside the refrigerated chamber of the shipping containerproduce permanent damage to conventional preferred electrochemicalanalyzers. These analyzers also suffer from a 3% per degree Celsiustemperature coefficient. It is therefore desirable to locate theanalyzer sensor in the refrigerated chamber of the container. Therefrigerated cargo space of the container however, is undesirablebecause of problems of mechanical damage to the analyzer sensor and thatthe cargo is typically designed to fill this space fully. A preferredspace candidate therefor comprises the circulating fan ducting of therefrigerated container. However, this space is extremely limited. Italso has the problem that high pressure water streams are periodicallyemployed to clean and sterilize this area.

The present invention overcomes these problems by designing a combinedcarbon dioxide and oxygen sensor that is very compact and low cost. Thisenables a sealed oxygen-carbon dioxide sensor assembly that is smallenough to fit in the available ducting space.

A typical prior art infrared analyzer consists of a source, means forpulsating the source so that the infrared energy from the source can bedistinguished from background energy, a sample cell to contain theunknown gas sample, a means for selecting the wavelength to the areawhere the analyte has high absorption, and a detector to convert theinfrared energy to an electrical signal. The source typically consistsof an electrically heated element, in combination with a motor drivenshutter to develop the pulsating infrared energy. An alternate prior artinfrared source consists of an evacuated chamber with a low thermal masselectrically heated element and a window capable of transmitting thedesired wavelengths. This window is typically made of sapphire. For theparticular field of use of the present invention such prior art, motordriven source is too bulky and such prior art sapphire window evacuatedsource proves too expensive. Since the response time of the CO2 analyzercan be relatively slow, it was discovered in the present invention thata commercial available light bulb could be used to develop the pulsatingenergy if it is chopped at extremely low frequencies (less than 2 cyclesper second). The majority of the radiation with the source of thepresent invention is produced by the emission of the glass envelope. Thesensor of the present invention does not employ a sample cell in theclassical sense, but the entire optical path between the light bulb andthe detector is purged with the gas sample. In order to obtainreasonable life from the tungsten light bulb infrared source it isnecessary to employ current regulation of the energizing energy in orderto prevent the high initial current surge on turn-on. The light bulbssuitable for this application are similar to those used in flash lightsand are less than 10 watts. Since the sensor is located in therefrigerated section of the container which is temperature controlled,the temperature coefficient problems associated with the sensors aregreatly diminished.

SAMPLING SYSTEM

In order to draw sample and calibrant gases into the analyzer, a samplepump is typically employed in the prior art. These pumps have proven tobe both expensive and unreliable. To overcome this problem the largecirculating fans found in the refrigerated chamber are employed in thepresent invention to provide the suction needed to draw gas samples andcalibrants into the analyzer. By placing a sampling conduit within theair stream of the refrigeration systems circulation fan sufficientsuction is obtained. Where necessary a venturi or Pitot tube can beemployed to magnify this suction. The circulating fan is deenergizedduring the defrost cycle. For this reason it is necessary for themicroprocessor system to lock the controls and concentration displayduring this cycle and indicate that the readings are not current.

CALIBRATION SYSTEM

Oxygen and carbon dioxide analyzers require periodic calibration withgases of known concentration. In the application of use of the presentinvention, it is critical that the analyzers be accurate and functionalfor prolonged duration. The types and construction of the analyzersemployed in the present invention are selected to enable thiscalibration to be accomplished using outside air. In this regard, it isimportant to introduce calibrant gases not only for the purpose ofrecalibrating the analyzers but also to determine that they arefunctioning properly. Otherwise the produce stored in the container willspoil. The electrochemical oxygen sensor of the present inventionoperates by measuring the current produced by oxygen ions formed by thediffusion of oxygen across a membrane. The response is essentially alinear function of the percentage of oxygen in the sample cell. When thesensor fails or no oxygen is present, the output is zero. For thisreason, a single point upscale calibration is adequate to determine theproper functioning of the oxygen sensor. Outside air which is typically20.8% O₂ is sufficiently accurate for this purpose.

The carbon dioxide analyzer of the present invention is of the singlebeam type where the output signal drops to zero with no infrared energyand is a maximum with a nonabsorbing gas in the optical path. At aconstant temperature and with a fixed wavelength selection, the shape ofthe concentration versus signal does not change with this type ofanalyzer. Therefore, a single point calibration employing a nonabsorbinggas is satisfactory. Since outside ambient air typically is less than600 parts per million CO₂, ambient air can be employed for calibrationand determining that the CO2 analyzer is functioning properly.

MICROPROCESSOR CONTROL SECTION

The microprocessor of the present invention has a complex program whichconditions and operates system components pursuant to a priorityhierarchy chosen to insure optimum produce storage and to determine whenthese controlled devices should operate in order to maintain precise O2and CO2 levels in the refrigerated chamber.

In order to prevent overloading the limited power available in mobilshipping applications, the microprocessor prevents the atmosphericsystem from operating until the cool down cycle of the refrigerationsystem is complete and the refrigeration system is operating at reducedpower.

In order to optimize the performance of the membrane, the microprocessorcontrols the temperature of the inlet gas to the membrane. To controlthe concentration of the atmosphere in the refrigerated container themicroprocessor employs complex logic to decide when to operate themembrane bypass valve, the compressor and a CO2 solenoid valve. It alsoperiodically introduces air to calibrate the analyzers and to verifytheir performance. In the event the microprocessor detects an error inthe performance of the analyzers it automatically defaults to a safeoperational mode to maintain approximately correct atmospheric controlwithin the container.

More particularly, since some controlled parameters are more critical tothe preservation of the produce than others, it is necessary for thecontroller of the present invention to give priority to the mostcritical parameters when a conflict exists. The refrigeration system isthe most critical and is given preference when necessary to preventexceeding the peak power permitted. The control of oxygen is second inpriority, and the control of carbon dioxide is the least. When thecarbon dioxide level in the container exceeds the maximum desired, it isnecessary to dilute the same with nitrogen from the membrane system byopening the membrane bypass valve and turning on the compressor toproduce a maximum carbon dioxide dilution rate. This, however,simultaneously introduces gas having higher oxygen concentration. Ifthis action causes the oxygen level to exceed the maximum controlconcentration, the bypass valve is closed giving priority to controllingthe O2 level. If the CO₂ level is lower than the minimum set point, thecontroller opens the valve to a source of compressed CO₂ therebyenriching the CO₂ level in the container until the CO₂ levelrequirements are satisfied.

When the oxygen level exceeds the maximum set point, the compressor isturned on and the bypass valve is closed to introduce gas into thechamber having very low oxygen content. When the oxygen level is lowerthan the minimum set point, the compressor is turned on and the bypassvalve is opened to introduce gas into the chamber having an elevatedoxygen content. When the oxygen level is within the set limits, thecompressor is turned off unless needed for CO2 corrections.

In addition the microprocessor periodically operates a three-way valveto introduce outside air to the analyzer section. The microprocessorchecks the signals received from the analyzer during calibration todetermine if they are within the expected range. If they are not, themicroprocessor goes into a default mode for the failed analyte.Otherwise, it recalibrates to the correct values for air. This procedurechecks for correct analyzer and sampling performance. The oxygen sensortypically decreases its signal output with prolonged use. Themicroprocessor gives a warning, informing the operator that the sensorshould be replaced before the next trio when its output drops below aprogrammed value. When a default mode is detected for the oxygen sensor,the compressor operates full time and the bypass valve is closed. Thiscan produce an oxygen level lower than desired within the container, butthe adverse effect on the produce is less severe than exposure toelevated oxygen levels.

When the CO₂ level is lower than the minimum set point, themicroprocessor opens the CO₂ valve to introduce additional CO₂ into thechamber. When it exceeds the maximum set point, it takes the actiondescribed earlier. In the event the calibrant signal from the CO₂analyzer indicates CO₂ analyzer failure, then the CO₂ valve is closed.Low values of CO2 are less damaging to the produce than elevated values.

In addition, the microprocessor periodically operates the valve on awater trap to empty it. The conventional microprocessor further controlsthe inlet temperature to the membrane via a thermistor. Upon failure ofthe temperature sensor, the microprocessor defaults to a heater cycleprogram typically required to maintain operational temperature controlfor the membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

These, as well as other features of the present invention will becomemore apparent upon reference to the drawings wherein:

FIG. 1 is a schematic piping and control diagram of the presentinvention;

FIG. 2 is a perspective view of an alternative embodiment of the vacuumdelivery means used in the present invention;

FIG. 3 is a left side perspective view of the combined carbondioxide/oxygen sensor of the present invention;

FIG. 4 is a right side perspective view of the carbon dioxide/oxygensensor of the present invention;

FIG. 5 is an exploded view of the sensor shown in FIG. 4;

FIG. 6 is a cross-sectional view taken along line 6--6 of FIG. 5;

FIG. 7 is a cross-sectional view taken along line 7--7 of FIG. 5; and

FIG. 8 is a cross-sectional view taken along line 8--8 of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein the showings are for purposes ofillustrating a preferred embodiment of the present invention only, andnot for purposes of limiting the same, FIG. 1 schematically illustratesa system for maintaining a controlled atmosphere within a sealedcontainer 10. In the preferred embodiment, container 10 is constructedin a manner so as to be substantially air-tight and is refrigerated viaa conventional refrigeration system 12 which is fluidly coupled thereto.Additionally, disposed within the container 10 is at least onecirculation fan 14 for the refrigeration system 12 which is typicallydisposed within the conventional air duct (not shown) of the container10 and is used to maintain a uniform mixture of processed air within thecontainer 10.

The controlled atmosphere system as interfaced to the container 10comprises two independent gas flow paths. The first path is adapted todraw ambient air from outside the container 10 and, after processing ofthe same, subsequently introduce such processed air into the container10. The second path is adapted to draw a quantity of the processed airfrom inside the container 10 for sampling and analyzer calibrationpurposes. Importantly, each of the two flow paths comprise componentswhich are electrically interfaced to the controller 16 which is adaptedto selectively activate and deactivate each of the two flow paths and tocoordinate their operation.

AIR INLET SYSTEM STRUCTURE AND OPERATION

As depicted in FIG. 1, the flow path of the present invention whichdraws air from outside of the container 10 and subsequently introducessuch air into container 10 generally comprises a first inlet 18 which isadapted to receive ambient air from outside of container 10. Preferablydisposed within container 10 is an air compressor 20 which iselectrically interfaced to controller 16 and includes an intake portfluidly coupled to the first inlet 18. When controller 16 activatescompressor 20, ambient, i.e. outside air is drawn into the first inlet18. Positioned between the first inlet 18 and compressor 20 is a firstfilter 22 which is operable to remove salt mist or other particulatesfrom the air drawn into first inlet 18 before such air enters aircompressor 20. Ambient air drawn into compressor 20 through its intakeport is compressed to a typical value of approximately 100 PSI.

After the air is compressed by the compressor 20, the air is preferablycommunicated back outside of the container 10 and into a gas separationmeans 24 which includes an entrance port connected to the exhaust portof the compressor 20. In the preferred embodiment, the gas separationmeans 24 comprises a hollow fiber oxygen enricher which includes aplurality of hollow fiber permeable membranes 26 disposed therein.Importantly, when compressed air enters the bores of the hollow fiberpermeable membranes 26, fast traveling gases such as oxygen and carbondioxide as well as water are able to permeate through the walls of themembranes 26 at a faster rate than slower traveling gases such asnitrogen. Thus, as will be recognized, the rate at which air passesthrough the membranes 26 determines the volume and purity of thenitrogen which is produced by and exits through the exit port of the gasseparation means 24. The permeable membranes 26 utilized in conjunctionwith the present invention are manufactured by Permea, Inc. and are morethoroughly disclosed in U.S. Pat. No. 4,880,441 issued to Kesting etal., the disclosure of which is expressly incorporated herein byreference.

Disposed between air compressor 20 and gas separation means 24 is asecond filter 28. Second filter 28 is used to purify the air exhaustingfrom compressor 20 to 3.0 microns and to remove water vapor therefrom.Connected to second filter 28 is a sump 30 which is used to collectwater removed from the compressed air by the second filter 28.Additionally, connected to sump 30 is a first solenoid valve 32 which iselectrically interfaced to controller 16. First solenoid valve 32 isinterfaced to a drain (not shown) disposed within sump 30 and isperiodically activated by controller 16 to drain water which hasaccumulated within the sump 30. Disposed between the second filter 28and gas separation means 24 is a heater 34 which is also electricallyinterfaced to controller 16. Heater 34 is operable to heat thecompressed air to a temperature set point of approximately 50 degreesCelsius before such air enters gas separation means 24. Attached to theheater 34 is a thermistor 36 which is also electrically interfaced tocontroller 16 and operable to sense the air temperature and send an airtemperature signal to the controller 16.

Connected to the exit port of the gas separation means 24 is a firstoutlet 38. First outlet 38 is used to place the exit port of the gasseparation means 24 in fluid communication with processed air inside thecontainer 10. Connected to the first outlet 38 is a first valve 40 whichis used to regulate the flow of air through gas separation means 24.Also connected to the first outlet 38 is a bypass channel 42incorporating a second solenoid valve 44 which is electricallyinterfaced to the controller 16, and a restricting orifice 46. As seenin FIG. 1, bypass channel 42 is interfaced to first outlet 38 in amanner whereby bypass channel 42 is operable to form a flow path betweengas separation means 24 and container 10 which does not include thefirst valve 40. As such, when the second solenoid valve 44 is notactivated, gas which exits gas separation means 24 and flows into firstoutlet 38, flows only through first valve 40 before entering container10. When second solenoid valve 44 is activated, gas exiting gasseparation means 24 flows through two separate flow paths. The firstpath is the same as previously described wherein the gas enters firstoutlet 38 and flows through first valve 40 before entering container 10.In the second path, after gas enters first outlet 38, the gas flows intobypass channel 42, through restricting orifice 46, and back into firstoutlet 38 before entering container 10. As can be appreciated, whensecond solenoid valve 44 is activated, the flow of air through gasseparation means 24 is increased. In contrast, when second solenoidvalve 44 is not activated, the flow of air through gas separation means24 will be relatively slow since all of the air channeled into firstoutlet 38 must pass through first valve 40. In those instances when theflow rate of air through gas separation means 24 is slow, greateramounts of oxygen and carbon dioxide are allowed to permeate throughpermeable membranes 26, thereby causing a gas which is very low inoxygen and carbon dioxide content to be introduced into the container 10via first outlet 38. When second solenoid valve 44 is opened, the flowrate of air through gas separation means 24 significantly increases,since in addition to traveling through first valve 40, gas exiting gasseparation means 24 also travels through bypass channel 42, andrestricting orifice 46. As such, the increased flow rate through gasseparation means 24 causes less oxygen to be removed from the air,thereby causing gas having increased oxygen levels to be introduced intothe container 10. Thus, by manipulating the first valve 40 and thesecond solenoid valve 44 in a desired manner, two different flow ratesthrough gas separation means 24 may be achieved, thereby allowing gashaving increased or decreased concentrations of oxygen to be introducedinto the container 10. Additionally, those skilled in the art willrecognize that the bypass channel can alternatively be arranged toincrease air flow into the container directly from filter 28 therebybypassing the membranes 26.

Also fluidly coupled to first outlet 38 is a third solenoid valve 48which is electrically interfaced to controller 16. Connected to thirdsolenoid valve 48 is a tank 50 containing pure carbon dioxide. Whenthird solenoid valve 48 is actuated by the controller 16 to an openposition, pure carbon dioxide is allowed to pass from the tank 50 intothe first outlet 38 and subsequently into the container 10.

GAS SAMPLING SYSTEM STRUCTURE AND OPERATION

The second flow path of the present invention consists of a samplingsystem which is used to monitor the oxygen and carbon dioxide levels ofgas within the refrigerated container 10. Disposed within the contained10 is a sample inlet 52 which is adapted to receive gas samples fromtherewithin. Fluidly coupled to one end of the sample inlet 52 is a gasanalyzing means 54 which is also disposed directly within the container10 and is used for sensing the oxygen and carbon dioxide levels of gaswithin the container 10. As will be described in more detail, in thepreferred embodiment, the gas analyzing means 54 incorporates a combinedcarbon dioxide sensor 56 and an oxygen sensor 58 which are arranged in aseries flow path. In the preferred embodiment, the carbon dioxide sensor56 comprises an infrared sensor, while the oxygen sensor 58 comprises anelectrochemical transducer. Since high temperatures or extremetemperature fluctuations adversely affect the performance of the carbondioxide and oxygen sensors 56, 58, disposing the gas analyzing means 54within the interior of the refrigerated container 10 subjects thesensors 56, 58 to constant temperatures when operating, thus enhancingthe performance thereof. Additionally, the gas analyzing means 54 isfabricated so as to be liquid and vapor tight thus protecting the carbondioxide and oxygen sensors 56, 58 therewithin from exposure tofungicides, cleansers or other agents which are typically andperiodically applied to the interior of the refrigerated container 10under high pressure for purposes of removing bacteria, spores and otherelements from the walls thereof.

The second air flow path of the present invention further comprises asuction or vacuum delivery means which is disposed within the container10 and is fluidly coupled to the gas analyzing means 54 for selectivelydrawing a gas sample through the sample inlet 52 and the gas analyzingmeans 54. In the preferred embodiment, the delivery means comprises atube 60 having a first end fluidly coupled to the gas analyzing means 54and a second end located to be disposed within the air stream A of thecirculation fan 14 disposed within the container 10. The tube 60 maycomprise a conventional pitot tube (shown in FIG. 1) a pair of pair oftubes 68 (illustrated in FIG. 2) or a venturi tube arrangement (notshown) when increased suction is desired. As will be recognized, thepassage of the air stream A over the second end of the tube 60 or tubes66 when the circulation fan 14 is energized causes a vacuum to be pulledwithin the tube 60 or tubes 66 thus drawing a gas sample from thecontainer 10 into the gas analyzing means 54 via the sample inlet 52.The vacuum delivery means further includes a control valve 64 which isfluidly coupled between the first end of the tube 60 and the gasanalyzing means 54 and is electrically interfaced to the controller 16.In the preferred embodiment, the control valve 64 is selectivelyactuatable between open and closed positions by the controller 16 and isused for purposes which will be described below. As will be recognized,the gas sample, after being drawn through the gas analyzing means 54 andcontrol valve 64, is reintroduced into the interior of the container 10via the open distal tips 62 of the tube 60 or pair of tubes 66.

Advantageously, the use of the tube 60 or tubes 66 disposed within thecirculation fan airstream eliminates the necessity of having to usesmall sample pumps in the sampling system which, as previouslyindicated, are known to be unreliable and have relatively short lifespans, particularly when exteriorally mounted and employed in salt waterenvironments. In the present invention, the circulation fan 14, whenenergized, produces high velocity air flow within the container 10 inthe range of 400 to 500 CFM. As such, the placement of the second end ofthe tube 60 or tubes 66 in the direct flow of air above or below the fanblade of the circulation fan 14 produces reliable suction required todraw the gas sample from within the interior of the container 10 intothe gas analyzing means 54 via the sample inlet 52.

CALIBRATION PROCEDURE

In the present invention, both the carbon dioxide sensor 56 and oxygensensor 58 of the gas analyzing means 54 utilize periodic calibration toinsure accuracy and proper operation of the sensors. In the preferredembodiment of the present invention, outside, ambient air is employedfor calibration. The use of ambient air for calibration purposes isextremely advantageous since standardized calibrant gas supplies arecostly and/or unavailable in remote geographic locations, and alsooftentimes possess varying purity levels. Fluidly coupled within thesample inlet 52 is a fourth solenoid valve 72 which is electricallyinterfaced to the controller 16 and selectively actuatable between firstand second positions. Extending through the wall of the container 10 isan ambient air inlet 74 having a first end fluidly coupled to the fourthsolenoid valve 72 and a second end which is disposed outside of thecontainer 10. In the preferred embodiment, the fourth solenoid valve 72is adapted to create an open passage between the sample inlet 52 and gasanalyzing means 54 when in the first position and an open passagebetween the ambient air inlet 74 and gas analyzing means 54 when in thesecond position. During the air calibration procedure, actuation of thefourth solenoid valve 72 to the second position by the controller 16which occurs only when the circulation fan 14 is energized and theopening of the control valve 64 causes ambient air to be drawn into theambient air inlet 74 and through the gas analyzing means 54. Thereafter,the ambient air flows through the control valve 64 and pitot tube 60 ortubes 66 and subsequently into the interior of the container 10.Disposed within the ambient air inlet 74 on the outside of the container10 is a third filter 76 which is operable to remove any contaminantsfrom the ambient air entering the ambient air inlet 74 before such airis able to pass through the fourth solenoid valve 72 and into the gasanalyzing means 54. Importantly, the introduction of the relativelysmall amounts of ambient air which are needed for the calibrationprocedure into the interior of the container 10 does not degrade thecontrolled atmosphere established therewithin.

The sampling of gas within the interior of the container 10 and thecalibration of the gas analyzing means 54 with ambient air from outsideof the container 10 is conducted automatically by the controller 16. Inthe preferred embodiment, measurements of the oxygen and carbon dioxidelevels within the container 10 are continuously taken via actuation bythe controller 16 of the fourth solenoid valve 72 to the first positionand the actuation of the control valve 64 to the open position. Aspreviously indicated, the sampling procedure only occurs when thecirculation fan 14 is energized since the air stream A created therebyis needed to create a vacuum within the tube 60 or tubes 66.

The air calibration procedure is conducted by the controller 16 via theactuation of the fourth solenoid valve 72 to the second position and theactuation of the control valve 64 to the open position. Additionally, aspreviously explained with respect to the air sampling procedure, thecirculation fan 14 must also be energized so as to create a vacuumwithin the tube 60 or tubes 66 to cause ambient air to be drawn into thegas analyzing means 54 via the ambient air inlet 74. In the preferredembodiment, the air calibration procedure is performed automatically bythe controller 16 approximately once per hour or manually as desired byactivation of the calibration switch control button 86 located on thehousing of the controller 16.

After allowing time for the carbon dioxide sensor 56 and oxygen sensor58 to be thoroughly purged with ambient air, the controller 16 assumes aproper baseline reading for the sensors to be 0.05 percent carbondioxide and 20.8 percent oxygen. These readings are used as baselinesettings for the next hour or similarly desired period. Importantly, theassumed carbon dioxide and oxygen levels coincide with the oxygen andcarbon dioxide levels typically found in ambient air. Since both the aircalibration procedure and gas sampling procedure require the circulationfan to be energized, the controller 16 is programmed to actuate thecontrol valve 64 to the open position only when the circulation fan 14is operating. Additionally, the controller 16 is programmed to onlyenergize the compressor 20 when the refrigeration system 12 is at areduced load. Thus, in the present system, the refrigeration system 12never operates at full capacity concurrently with compressor 20, nor dothe air calibration or gas sampling procedures occur when therefrigeration system 12 is operating under full load conditions. Assuch, the peak electrical load requirements of the controlled atmospheresystem of the present invention are greatly reduced.

As previously specified, during the air calibration procedure the gasanalyzing means 54 is calibrated based on the expected oxygen and carbondioxide concentrations of ambient air passing therethrough. In thepreferred embodiment the controller 16 is adapted to generate an alarmif, during the calibration procedure, the carbon dioxide and oxygensensors 56, 58 signals are outside expected parameters. When such analarm is generated by the controller 16, the control settings of thesensors 56, 58 are caused by the controller 16 to automatically defaultto predetermined control settings that approximate desired operationalsettings. The controller 16 is also programmed to allow the gasanalyzing means 54 to be completely purged with ambient air prior toconducting a calibration procedure and to determine whether such purginghas occurred by taking multiple readings of the oxygen and carbondioxide levels of the air which are separated in time.

In this respect, the controller 16 preferably stores such readings anddetermines whether the differences between consecutive readings arewithin accepted parameters. This comparison between consecutive readingsadditionally facilitates correction for signal inaccuracies caused byintermittent noise and/or power surges encountered in the electricalpower system for the container. In the preferred embodiment, pluralconsecutive readings such as five consecutive readings are reviewed bythe microprocessor and compared with the preceeding signal reading todetermine any trend or drift between the readings. If the trend or driftis within specified parameters, then the average signal readings or lastsignal reading is utilized and displayed by the microprocessor.Alternatively, if the trend or drift is outside of specified parameters,for instance if a noise spike is present on one of the consecutivereadings, then the noise spike is replaced with a subsequent signalreading and is thereby ignored by the microprocessor for display andcontrol purposes. As such inaccuracies caused by intermitten powerspikes and noise is eliminated in the present invention.

CONTROLLER STRUCTURE AND OPERATION

In the preferred embodiment of the present invention, the controller 16preferably comprises a housing 76 having a conventional processor suchas a microprocessor 77 disposed therein. The microprocessor 77 iselectrically interfaced to the following devices which serve as inputsto the microprocessors: namely the container refrigeration system 12 forrecognizing both defrost and reduced load signals, the thermistor 36;the analyzing means 54 and thus the carbon dioxide sensor 56 and oxygensensor 58 and the plural control switch buttons 80, 82, 84 and 86 asdescribed infra. Additionally the microprocessor 77 is electricallyinterfaced to the following device which serve as outputs for themicroprocessosr 77 namely the air compressor 20; first solenoid valve32; heater 34; second solenoid valve 44; third solenoid valve 48; fourthsolenoid valve 72; control valve 64 and the display and storage 78.

As shown in FIG. 1, display panel 78 as well as the four functioncontrol buttons 80, 82, 84 and 86 are disposed upon the front of thehousing 76. More particularly, these function control buttons comprisean "UP" button 80, a "DOWN" button 82, a "SET" button 84 and a"FUNCTION" button 86. The UP button 80 and the DOWN button 82 are usedto adjust values or trigger events which are displayed on display panel78 through the use of the SET button 84 and the FUNCTION button 86. TheSET button 84 scrolls through choices that let the user set thefollowing list of values:

    ______________________________________                                        ITEM             RANGE       DEFAULT                                          ______________________________________                                        Membrane Gas Input                                                                             (40-60° C.)                                                                        50° C.                                    Temperature                                                                   Oxygen Set Point (0.0-20.0%) 5.0% Oxygen                                      Oxygen           (0.1-1.0%)  1.0% Oxygen                                      Hysteresis                                                                    Carbon Dioxide   (0.0-20.0%) 5.0% Carbon                                      Set Point                    Dioxide                                          Carbon Dioxide   (0.1-1.0%)  1.0% Carbon                                      Hysteresis                   Dioxide                                          ______________________________________                                    

The FUNCTION button 86 also allows the user to manually initiate acalibration cycle. The user can also display on display panel 78 thetotal compressor on time, and by using a combination of theaforementioned buttons, clear the total time. The total compressor ontime is displayed on display panel 78 in hours with the maximum valuebeing 4660 hours. If the maximum is exceeded, the count changes to theword "OVERFLOW" on the display panel 78 until the count is cleared backto zero.

CONTROL LOGIC AND SYSTEM OPERATION

Referring now to FIG. 1, during system operation, the oxygen levelwithin container 10 is typically maintained at a level of two (2) tofive (5) percent while the carbon dioxide level is typically maintainedat a level of one (1) to ten (10) percent depending on the type ofproduce, meat or other perishable commodity being stored within therefrigerated container 10. The remainder of the gas within the container10 preferably comprises nitrogen and water vapor. In the initial phasesof operation, the carbon dioxide sensor 56 and oxygen sensor 58 of thegas analyzing means 54 are calibrated using the air calibrationprocedure in the manner previously described. These calibration valuesare then stored within the microprocessor of the con-roller 16 andsubsequently used for comparison purposes of sensor signals obtainedduring continuous gas sampling.

During operation of the refrigeration circulating fan, themicroprocessor within the controller 16 causes a gas sample to be drawninto the gas analyzing means 54 in a manner previously described, toallow measurements of the carbon dioxide and oxygen levels within thecontainer 10 to be taken by the oxygen and carbon dioxide sensors 56,58. The controller 16 evaluates the signals generated by the gasanalyzing means 54 in a manner previously described and determineswhether such levels are too low or too high relative desired oxygen andcarbon dioxide level set points or bands. If desired, such oxygen andcarbon dioxide measurement may be optionally dumped to a RAM memoryboard with a back up memory controlled by the microprocessor. The oxygenand carbon dioxide set point levels are typically two percent and fivepercent, respectively, as inputted into the microprocessor by thefunction control buttons, though it will be appreciated that otheroxygen and carbon dioxide set points may be programmed into themicroprocessor as different commodities have different requirements.Additionally, as previously specified, the acceptable oxygen and carbondioxide bands are typically two to five percent and one to ten percent,respectively.

Oxygen Level High

In response to a determination from the oxygen sensor 58 that the oxygenlevel within the container 10 exceeds a predetermined oxygen range, i.e.exceeding the two percent set point or not within the acceptable band,the microprocessor within the controller 16 responds by switching on theair compressor 20, and heater 34. Controller 16 will also maintainsecond solenoid valve 44 in the OFF position. Due to the activation ofthe air compressor 20, air will be drawn from outside of the container10 into the first inlet 18, and will subsequently pass into gasseparation means 24. Since second solenoid valve 44 is not activated,gas exiting gas separation means 24 into the first outlet 38 will passonly through first valve 40 before entering the container 10. For thereasons previously discussed, this particular flow path causes reducedflow through gas separation means 24, thereby permitting greaterquantities of oxygen to be removed from the air passing through thepermeable membranes 26. As such, gas entering container 10 through firstoutlet 38 has a greatly reduced oxygen level which therefore reduces theoxygen content within the container 10.

Oxygen Level Low

In response to a determination from the oxygen sensor 58 that the oxygenlevel within the container 10 is below a predetermined oxygen range,i.e. less than two percent, the microprocessor within the controller 16will once again actuate operation of the air compressor 20 and willlikewise activate the heater 34. Additionally, the controller 16 willalso turn the second solenoid valve 44 to the ON position. Onceactivated, air compressor 20 will draw ambient air into first inlet 18and channel such air into gas separation means 24. However, since secondsolenoid valve 44 is now activated, gas exiting gas separation means 24into the first outlet 38 will pass through restricting orifice 46 inaddition to the first valve 40. Thus, as previously discussed anddescribed, through the activation of second solenoid valve 44, the flowthrough gas separation means 24 will be greatly increased. Due to thisincreased flow, less oxygen will be removed from the air by permeablemembranes 26. Thus, the oxygen content of gas entering container 10 viafirst outlet 38 is increased, thereby increasing the oxygen levelswithin the container 10.

Carbon Dioxide Level High

In response to a determination from the carbon dioxide sensor 56 thatthe carbon dioxide level within the container 10 exceeds a predeterminedcarbon dioxide range, i.e. exceeding five percent or not within theacceptable band, the system will function in exactly the same manner aspreviously discussed with respect to the determination that the oxygenlevel within container 10 exceeds a predetermined oxygen range, therebydiluting the CO2 level within the container. This operating conditionwill continue to exist until such time as the oxygen level within thecontainer exceeds its maximum limit.

Carbon Dioxide Level Low

In response to a determination from the carbon dioxide sensor 56 thatthe carbon dioxide level within the container is below a predeterminedcarbon dioxide range, i.e. less than five percent, the microprocessorwithin the controller 16 will cause the third solenoid valve 48 toactuate to the open position which allows pure carbon dioxide to passfrom the tank 50 through first outlet 38 and into the container 10,thereby raising the carbon dioxide levels within container 10.

In the preferred control logic of the present invention, themicroprocessor is programmed with a time-based lock-out that preventsthe compressor 20 from being "short-cycled." Thus, the compressor 20will always be kept off for a minimum time of three minutes, even if thecontrol logic attempts to cause compressor activation to occur.Additionally, the temperature control facilitated by the thermistor 36is interlocked with the control of the compressor 20. Thus, preferablyno heating of the air will take place when the compressor 20 is notoperating. It is also contemplated that hardware circuitry may beutilized in conjunction with the present system that will detect afailed temperature sensor. If this condition occurs, the heater 34 willrun at a fixed duty cycle in an attempt to maintain a useful systemtemperature. An additional interlock may also be added to prevent thecontrolled atmosphere system of the present invention from operatinguntil the refrigeration system 12 has brought the container 10 to adesired temperature operation. This prevents the present system fromadding to the peak electrical power load. Such prevention is importantsince mobile installations have limited electrical power available.

GAS ANALYZING MEANS STRUCTURE AND OPERATION

Referring now to FIGS. 3-8, the preferred embodiment of the gasanalyzing means 54 used in the present invention is depicted. Theanalyzing means generally comprises a housing 100 defining a firstinterior chamber 102 and a second interior chamber 104, each of whichhave generally cylindrical configurations. The first chamber 102 definesa bottom surface 106, while the second chamber 104 defines a bottomsurface 108. As best seen in FIGS. 5 and 8, the second chamber 104 isformed to define an annular recess 110 in close proximity to the bottomsurface 108 thereof into which is received an O-ring 112. Formed aboutthe upper rims of the first and second chambers 102, 104, is acontinuous flange 114 which includes an open channel 116 extendingbetween the central portion thereof. Formed on and extending outwardlyfrom the outer surface of the upper portion of the housing 100 arerounded extensions 118 which include threaded apertures 120 extendingaxially therethrough. Releasably attachable to the housing 100 is a lidmember 122 which, as shown in FIG. 3, is formed in a manner such thatwhen placed over the flange 114, fasteners such as screws 124 may beextended therethrough into the apertures 120 of extensions 118. Whenassembled, the lid member 122 forms a liquid and vapor tight sealagainst the housing 100, thus preventing any moisture from enteringeither the first chamber 102 or second chamber 104.

Formed within the housing 100 is a gas sample inlet passage 126 and agas sample outlet passage 128, both of which are best seen in FIGS. 5and 6. In the preferred embodiment, the first end of the inlet passage126 opens into a threaded inlet aperture 130, while the second end ofthe inlet passage 126 opens into the bottom surface 108 of the secondchamber 104. Similarly, the first end of the outlet passage 128 opensinto the bottom surface 108 of the second chamber 104, while the secondend opens into a threaded outlet aperture 132. As seen in FIGS. 3 and 4,threadably received into the inlet aperture 130 is an inlet fitting 134,while threadably received into the outlet aperture 132 is an outletfitting 136. As will be recognized, gas samples enter the housing 100via the inlet fitting 134 and flow through the inlet passage 126 intothe lower portion of the second chamber 104. The sample gas then flowsfrom the lower portion of the second chamber 104, into and through theoutlet passage 128, with the gas sample eventually exiting the housing100 via the outlet fitting 136.

An elongate cavity 138 is formed in the bottom surface 106 of the firstchamber 102. As seen in FIGS. 6 and 7, cavity 138 is formed in thebottom surface 106 in an orientation wherein the cavity 138 communicateswith a portion of the inlet passage 126 via an aperture disposed in theside surface thereof. Formed on the lower portion of the housing 100 isan elliptically shaped side surface 140 which is circumvented by acorrespondingly shaped flange 142. The side surface 140 and flange 142define a side chamber 144 which is formed in close proximity to thecavity 138 and partially overlaps the first chamber 102. Formed in thelower portion of the side surface 140 is an annular recess 146.Additionally, formed in the bottom of the annular recess 146 is a sideaperture 148 which extends through the housing 100 and communicates withthe inlet passage 126. In the preferred embodiment, the annular recess146 is formed in the side surface 140 in an orientation such that theside aperture 148 formed in the bottom thereof will communicate with theinlet passage 126 and be in coaxial alignment with the aperture disposedin the side surface of the cavity 138. As such, the housing 100 of thegas analyzing means 54 is formed such that a continuous passage existsbetween the interior of the cavity 138 and annular recess 146 with suchpassage being bisected by the inlet passage 126. Disposed in the upperportion of the side surface 140 is an upper aperture 149 which extendsthrough the housing 100 and communicates with the lower portion of thefirst chamber 102. In this respect, the upper aperture 149 forms an openpassage between the side chamber 144 and the first chamber 102.

Releasably attachable to the bottom surface 106 of the first chamber 102is an annular socket plate 150. In the preferred embodiment, theattachment of the socket plate 150 to the bottom surface 106 isfacilitated by a pair of fasteners such as screws 152 which are extendedthrough a pair of apertures 154 disposed within the socket plate 150 andsubsequently received into a corresponding pair of threaded apertures156 disposed within the bottom surface 106 of the first chamber 102.Attached to the lower surface of the socket plate 150 is an infraredlight source 158. In the preferred embodiment, the light source 158consists of an incandescent light, and more particularly, a GILWAY L6402bulb. The light source 158 is attached to the lower surface of thesocket plate 150 such that the electrical leads 160 thereof extendupwardly through the socket plate 150 in the manner shown in FIGS. 5 and7. Additionally, the light source 158 is attached to the lower surfacein an orientation wherein the light source 158 will be received withinthe cavity 138 when the socket plate 150 is attached to the bottomsurface 106, in the manner shown in FIG. 7. In this respect, the cavity138 is sized so as to define an open cavity about the entire outersurface of the light source 158 when such is received thereinto. In thepreferred embodiment, an annular rubber seal 162 having a sizeapproximately equal to the size of the socket plate 150 is disposedbetween the socket plate 150 and the bottom surface 106 of the firstchamber 102. The tightening of the screws 152 causes the rubber seal 162to be compressed between the socket plate 150 and the bottom surface106.

Disposed within the annular recess 146 is an optical filter 164. As bestseen in FIG. 7, the optical filter 164, when placed within the annularrecess 146, resides on the shoulder defined by the intersection of therecess 146 and the side aperture 148. Also disposed within the annularrecess 146 is a pyroelectric infrared sensor 166. In the preferredembodiment, the infrared sensor 166 is sized such that when fullyreceived into the annular recess 146, the top surface thereof will besubstantially flush with the side surface 140. As will be recognized,when the optical filter 164 and infrared sensor 166 are disposed withinthe recess 146, the filter 164 will completely shield the sensor 166from the side aperture 148. To protect the infrared sensor 166 from thederogatory effects of any fungicides or other cleansers sprayed into theinterior of the container 10, a cover 168 is releasably attached to theelliptical flange 142 via a fastener 170 which is extended through anaperture 172 disposed within the cover 168 and received into a threadedaperture 174 disposed within the central portion of the side surface140. Though not shown, when the cover 168 is engaged to the flange 142,the electrical lead wires extending from the sensor 166 extend throughthe enclosed side chamber 144 and into the first chamber 102 via theupper aperture 149.

Disposed within the second chamber 104 is an oxygen sensor 176. As seenin FIGS. 5 and 8, the oxygen sensor 176 includes a gas inlet 178 formedon an extending downwardly from the bottom surface thereof. In thepreferred embodiment, the oxygen sensor 176 is disposed within thesecond chamber 104 in a manner wherein the O-ring 112 is firmlycompressed between the annular recess 110 and the bottom surface of theoxygen sensor 176. When the oxygen sensor 176 is disposed within thesecond chamber 104 in this manner, an annular circulation space orchamber 180 is defined between the gas inlet 178 of the oxygen sensor176 and the bottom surface 108 of the second chamber 104. As best seenin FIG. 8, the circulation chamber 180 is sealed via the compression ofthe O-ring 112 against the bottom surface of the oxygen sensor 176 whichoccurs when the lid member 122 is attached to the flange 114. Extendingoutwardly from the upper portion of the oxygen sensor 176 are itsassociated lead wires 182.

As best seen in FIG. 4, the oxygen sensor 176 is disposed within thesecond chamber 104 in a manner wherein the electrical lead wires 182extend through the channel 116 defined by the flange 114 and into thefirst chamber 102. Advantageously, the inclusion of the channel 116allows the wires 182 to pass into the first chamber 102 though the lidmember 122 which is rigidly attached to and sealed against the flange114. Due to the inclusion of the electrical leads 162 of the lightsource 158 on the top surface of the socket plate 150, the extension ofthe wires of the sensor 166 through the upper aperture 149 and theextension of the wires 182 through the channel 116, the first chamber102 may be used as an electrical lead junction box when the lid member122 is attached to the housing 100. After the proper electricalconnections are made within the first chamber 102, one or more wires maybe extended through a single sealed cable 184 which is attached to thetop surface of the lid member 122 in fluid-tight relation and isoriented over the first chamber 102 when the lid member 122 is attachedto the housing 100. As such, due to the fluid-tight engagement of thelid member 122 to the housing 100, the fluid-tight engagement of thecover 168 to the flange 142, and the passage of all the electrical wiresthrough the single sealed cable 184, the internal components on the gasanalyzing device 54 are protected from exposure to any liquids or vaporswhich may have derogatory effects on such components when the interiorof the refrigerated container 10 is cleaned.

As previously specified, the gas analyzing means 54 is used to measurethe carbon dioxide and oxygen levels of gas samples from within theinterior of the container 10. In operation, the pitot tube 60 or tubes66 as previously described are fluidly connected to the outlet fitting136, with the sample inlet 52 being fluidly connected to the inletfitting 134. When suction is created by the pitot tube 60 or tubes 66,the gas sample is pulled into the sample inlet 52 and introduced intothe inlet passage 126 via the inlet fitting 134. The gas sample flowsthrough the inlet passage 126 and past the passageway defined by theside aperture 148 and side wall aperture of the cavity 138. Thispassageway defines an optical path for the gas to be analyzed and as thegas sample passes therethrough the radiation from source 158 passesthrough the gas, optical filter 164 and contacts the infrared sensor166.

As is well known, infrared analyzers require a source of pulsinginfrared energy generated at a wavelength which is absorbable by the gasbeing tested. The infrared energy must be pulsed in order to distinguishbetween the radiation produced by the source and the steady radiationproduced by all surfaces at room temperature. Typically, with carbondioxide analysis, the required wavelength of energy is approximately 4.3microns. In the preferred embodiment of the present invention, theinfrared source 158 is pulsed i.e. chopped at a rate of 0.5 secondswhich provides a satisfactory frequency for the analysis of the carbondioxide levels within the gas sample. The light source 158 includes arelatively thin electrically nonconducive glass or quartz outer sheathor envelope which is periodically heated and cooled by a tungstenfilament disposed therein. Because glass and quartz do not pass infraredradiation, the infrared radiation is not produced by the heating of thetungsten filament therein. Rather, the infrared radiation is produced bythe glass or quartz sheath itself when heated by the passage of currentthrough the tungsten filament. Advantageously, the glass or fused quartzsheath of the light source 158 is thin enough such that the temperatureresponse is satisfactory for obtaining the desired radiationtransmission.

In addition to the glass or fused quartz sheath having good emissioncharacteristics for use in the cas analyzing means 54, the relativelylarge surface area of the sheath enables ample infrared energy to beproduced at much lower temperatures than are typically employed in priorart analyzers. Because the temperature coefficient of resistance ratioof the tungsten filament would result in large surge currents and shortlife of the source 158 if a regulated voltage was employed to energizethe source 158, current regulation is employed wherein the current isregulated at a fixed level which is substantially less than the normalworking voltage of the source 158. By using such current regulation andoperating the source 158 at less than its rated voltage, the life of thelight source 158 is typically extended to a time period greater than10,000 hours. When the source 158 is pulsed in the aforementionedmanner, infrared radiation is passed through the gas sample in theoptical path. The radiation then passes through the optical filter 164which eliminates interfering wavelengths and is received into theinfrared sensor 166 which essentially counts the number of absorbing gasmolecules to provide a measurement of the carbon dioxide level of thegas sample.

After the carbon dioxide level of the gas sample has been measured inthe aforementioned manner, the gas sample is subsequently introducedinto the circulation chamber 180 via the inlet passage 126. Thereafter,a portion of the gas sample enters into the oxygen sensor 176 via thegas inlet 178. As previously specified, the oxygen sensor 176 operateson an electrochemical principle based on the reduction of oxygen ions toproduce a current. Particularly, the oxygen of the gas sample diffusesthrough a membrane contained within the oxygen sensor 176 to anelectrolyte which forms an oxygen ion. The rate of diffusion is afunction of the concentration of the oxygen within the sample. Theoxygen ion is reduced at an electrode to produce an electrical currentthat is proportional to the oxygen concentration in the gas sample.After passing into the oxygen sensor 176, the gas sample passes from thecirculation chamber 180 into the outlet passage 128 which is in fluidcommunication therewith. Thereafter, the gas sample passes through theoutlet passage 128 and through the outlet fitting 136 into the pitottube 60 or tubes 66. As previously explained, the gas sample is thenreintroduced into the interior of the refrigerated container 10 via theopen distal tip 62 of the Pitot tube 60 or open distal tips 70 of thetubes 66.

As previously explained, to insure proper operation of the presentsystem, the gas analyzing means 54 must be periodically calibrated. Forsuch calibration to occur, both the infrared sensor 166 and oxygensensor 176 need to have gases of known concentrations of carbon dioxideand oxygen periodically introduced thereinto to check for properperformance and to provide a basis for recalibration if necessary. Ascan be appreciated, the oxygen analyzer 176 will produce no current inthe absence of oxygen ions. Therefore, the zero for the oxygen sensor166 is stable. However, the span varies with temperature, membranecondition and age.

In infrared analyzers, the energy of the optical beam is typicallyattenuated less than twenty percent (20%) by a full scale concentrationof the analyte. Prior art infrared analyzers typically employ eitherdual optical beams, dual sources, or dual detectors to balance thesignal to reduce the effects of changes in source emissions, opticalpath transmission, detector sensitivity, and amplifier gain. Anythingthat affects the balance of these analyzers also affects the outputsignal. For this reason, two point calibration using two calibrant gasesis typically required. For the infrared analyzer 56 of the presentinvention, a single calibrant gas is sufficient. The effect of changesin the infrared sensor 166 sensitivity, optical path 146 transmission,amplifier gain, and source 158 emission affect the zero reading. Byusing a calibrant gas having no absorption at the wavelengths theinfrared sensor 166 is responsive to, one point is established by zerosignal. The problems of stability are overcome by frequent calibrationusing ambient air as a calibrant gas. By setting limits for the outputsignal under these conditions the proper functioning of all the infraredanalyzer elements are checked. This provides means for determining theaccuracy and proper functioning of the infrared analyzer of the presentinvention.

As will be further recognized, by use of the gas analyzing means 54 ofthe present invention, the carbon dioxide and oxygen levels are measuredvia a series flow path with the carbon dioxide measurement initiallybeing taken and the oxygen level measurement subsequently being takendue to the flow pattern of the gas sample through the housing 100. Sinceboth the infrared sensor 166 and oxygen sensor 176 are adverselyaffected by temperature variations, the placement of the gas analyzingmeans 54 within the interior of the refrigerated container 10 provides alow, controlled temperature environment which facilitates properoperation of the gas analyzing means 54. However, since the interior ofthe container 10 is frequently washed with cleansing agents, the sensingcomponents of the gas analyzing means 54 must be protected from liquidand/or vapor penetration. As previously specified, the attachment of thelid member 122 to the flange 114 and the attachment of the cover 168 tothe flange 142 protects the sensing components from such exposure.Additionally, the use of the first chamber 102 as a junction box and theextension of a single sealed cable therefrom further protects thesensing components from exposure to liquid or vapor.

Additional modifications and improvements of the present invention mayalso be apparent to those skilled in the art. Thus, the particularcombination of parts described and illustrated herein is intended torepresent only one embodiment of the invention, and is not intended toserve as limitations of alternative devices within the spirit and scopeof the invention.

What is claimed is:
 1. A method for maintaining controlledconcentrations of oxygen and carbon-dioxide gas within a refrigeratedshipping container, said method comprising the steps of:(a) measuringthe carbon-dioxide and oxygen concentrations of gas within therefrigerated shipping container by utilizing a circulation fan of therefrigerated shipping container to effect the flow of gas, so as toobtain a sample thereof; (b) varying the concentrations ofcarbon-dioxide and oxygen within the shipping container in response tosuch measuring, so as to maintain controlled concentrations thereof; and(c) ceasing measurement of the carbon-dioxide and oxygen concentrationsof gas within the refrigerated shipping container when the aircirculation fan of the refrigerated shipping container is not operating.2. The method as recited in claim 1 wherein the step of measuring thecarbon-dioxide and oxygen concentration of gas within the refrigeratedshipping container by utilizing the air circulation fan comprises:a)drawing a gas sample from an inlet disposed within the refrigeratedshipping container, the gas sample being drawn into the inlet by lowpressure formed by the air circulation fan; b) communicating the gassample to carbon-dioxide and oxygen sensors; and c) sensing thecarbon-dioxide and oxygen concentrations of the gas samples.
 3. Themethod as recited in claim 1 wherein the step of utilizing the aircirculation fan to effect the flow of gas comprises utilizing suctiongenerated by the air circulation fan to effect the flow of gas.
 4. Themethod as recited in claim 1 wherein the step of utilizing the aircirculation fan to effect the flow of gas comprises utilizing a pitottube for generating suction to effect the flow of gas from within therefrigerated shipping container to the gas analyzer.
 5. The method asrecited in claim 1 wherein the step of utilizing the air circulation fanto effect the flow of gas comprises utilizing a pitot tube disposedwithin an air stream of the air circulation fan of the refrigeratedshipping container.
 6. The method as recited in claim 1 wherein the stepof measuring the carbon-dioxide and oxygen concentrations of gas withinthe refrigerated shipping container comprises measuring thecarbon-dioxide and oxygen concentrations via carbon-dioxide and oxygensensors located within the refrigerated shipping container, location ofthe carbon-dioxide and oxygen sensors within the refrigerated shippingcontainer providing a stable and controlled environment therefor.
 7. Themethod as recited in claim 1 wherein the step of utilizing the aircirculation fan to effect the flow of gas comprises utilizing a pair oftubes for generating suction to effect the flow of gas from within therefrigerated shipping container to the gas analyzer.
 8. The method asrecited in claim 1 wherein the step of utilizing the air circulation fanto effect the flow of gas comprises utilizing a pair of tubes disposedwithin an air stream of the air circulation fan of the refrigeratedshipping container.